There is a potential issue, it can result in other packages you want not having their requirements meant because they require the older version, those usually can be worked around (often just rebuilding the src.rpm so it links against the version you have) - but that's not generally recommended for people who just need things to work without having to futz around.
Yep, that's why I wouldn't do it with glibc or the kernel. The closest candidate would be MySQL, because I needed some v4 features and I knew the upgrade was going to break other things like PHP. Upgrading by repackaging something like that with lots of dependencies takes a bit of planning.
